An attrition mill is essentially a machine in which a rotating perforate drum receives material that is to be pulverized. In some applications, steel balls or other metallic objects are incorporated along with the material charged into the machine to perform the pulverizing. The steel balls remain in the rotating drum, and the pulverized particles below a given size pass through the perforations in the drum and into some sort of container. When the charge of material is a mixture of metallic and non-metallic substances, it is often unnecessary to use special pulverizing objects. The metal components of the charge material tend to remain integral masses, which function to pulverize the non-metallic substances to the point that the resulting particle size will pass through the openings in the rotating drum or grate. The term "grate" is particularly appropriate for one relatively standard form of structure in which the cylindrical configuration is formed by a stack of axially-spaced circular bars held together by bolts or by welding. The usual procedure for constructing such a component is to roll the bars individually to a circular configuration, and then secure them with bolts to form the generally cylindrical grate. The spacing between these bars corresponds to the desired particle size which the machine is expected to discharge. This spacing will commonly be on the order of a sixteenth to an eighth of an inch. It is difficult, if not impossible, to maintain exact similarity between individually-rolled bars, and any deviations distort the uniformity of the active interior surface of the grate.
An attrition mill with a grate of two to three feet in diameter and perhaps three to four feet long can easily receive a charge of material that may contain solid of chunks of enough mass to generate tremendous forces as the drum rotates. It is usually most convenient to charge the rotating drum through a peripheral opening which is then closed by a cover. Obviously, this cover, and the structure securing it, are subject to the same degree of heavy pounding action that is delivered to the interior surface of the rest of the rotating grate. It has proven to be rather difficult to provide a sufficient security of the cover without generating problems with regard to the time required to remove the cover and re-install it. The operation of these machines is a purely commercial operation, and the time required to charge the machine and manipulate the cover represent periods in which the machine is out of productive operation. The present invention is directed at establishing a method for constructing the rotating grate in a much more efficient manner than previous practice, and to providing a grate structure that establishes adequate retention of a cover together with simplicity in removing and re-installing it.